The invention relates to fabricating parts made of thermostructural composite material. A field of application for the invention is that of making structural parts for engines or thruster assemblies used in the fields of aviation and space.
Thermostructural composite materials are remarkable for their mechanical properties and for their ability to conserve these mechanical properties at high temperatures.
Well-known thermostructural composite materials are carbon/carbon (C/C) composite materials comprising carbon fiber reinforcement densified by a carbon matrix, and ceramic matrix composite (CMC) materials comprising fiber reinforcement made of refractory fibers (carbon or ceramic fibers) densified by a ceramic matrix or by a matrix that is essentially ceramic.
In a CMC material, it is well known that the presence of an embrittlement-relief interphase interposed between the fibers of the fiber reinforcement and the matrix greatly diminishes the sensitivity of the material to cracking and increases its ability to withstand impacts. The interphase is made of a material that is capable of relieving stresses at the bottoms of cracks that propagate through the matrix to the interphase, and that is thus capable of avoiding or delaying propagation of the cracks through the fibers. Such propagation of cracks through the fibers causes them to break, so preventing it makes the CMC material less fragile. The material constituting the interphase may be constituted for example by pyrolytic carbon (PyC) or boron nitride (BN) that is deposited on the fibers of the fiber reinforcement by chemical vapor infiltration (CVI), as described in particular in document U.S. Pat. No. 4,752,503. Another material that is suitable for an embrittlement-relief interphase is boron-doped carbon (BC).
In a C/C material, the presence of an interphase between the fibers of the fiber reinforcement and the carbon matrix, the interphase being of a material other than carbon, can also be useful in particular for improving oxidation behavior, e.g. if use is made of a boron-containing interphase, such as BN or BC.
Furthermore, in order to make a thermostructural composite material part that is complex in shape, it is known to begin by making a fiber preform that is to constitute the fiber reinforcement of the part by shaping a fiber structure and by keeping it in the desired shape by consolidation. Consolidation consists in densifying the fiber preform in part by means of a consolidating matrix phase, with this partial densification being performed so as to be sufficient, and preferably only just sufficient, to ensure that the preform can be handled while conserving its shape without assistance from support tooling. This enables densification of the consolidated preform to be continued without requiring tooling, and that is particularly advantageous, in particular when densification is continued by CVI in an oven, where otherwise the tooling would occupy a major fraction of the useful volume of the oven.
The fiber preform may be consolidated by CVI. The preform, while being held in the desired shape by tooling, is then placed in an oven where it is possible successively to form an interphase coating on the fibers and to consolidate them by partial densification. However, that returns to the above-mentioned drawback of tooling occupying useful space in the oven. In addition, and as is well known, CVI processes are lengthy, even for densification that is only partial.
It is therefore advantageous to perform consolidation by a liquid technique, that involves shaping the fiber preform from a fiber texture that has been impregnated with a liquid consolidation composition containing a resin that is a precursor of the carbon or ceramic consolidating matrix phase, the resin subsequently being transformed into a solid carbon or ceramic residue by pyrolysis. Nevertheless, prior formation on the fibers of the fiber texture of an interphase coating presenting sufficient thickness to perform the embrittlement-relief function has detrimental consequences on the ability of the fiber structure to be deformed, and that can make it impossible to put the fiber preform into the desired shape, particularly when the part that is to be made is of a shape that is relatively complex.
To solve that difficulty, proposals are made in document U.S. Pat. No. 5,486,379 to put an interphase into place after the fiber preform has been consolidated by a liquid technique and prior to continuing densification. Nevertheless, because of the presence of the consolidating phase, it is not possible to guarantee that the interphase will be formed continuously on the fibers of the fiber preform, in particular when the solid residue of pyrolyzing the consolidating resin adheres strongly to the fibers, as happens for example with the residue of pyrolyzing a resin that is a precursor of silicon carbide (SiC) on SiC fibers.